1. Field of the Invention
The present invention relates to a method of polarization-treating a piezoelectric material for use in ceramic filters, ceramic oscillators, and so forth.
2. Description of the Related Art
According to conventional methods of polarizing piezoelectric ceramics having a granular structure, typically in PZT type ceramics, desired polarization degrees are obtained by sputtering a silver electrode onto a firing material, and applying a direct voltage of about 2 to 5 kV/mm at a temperature ranging from room temperature to about 150xc2x0 C. for a time of from several seconds to several minutes. However, the conventional methods have the drawback that the characteristics of the piezoelectric ceramics are degraded by a thermal shock (by soldering and so forth) generated in a process after the polarization treatment. One of the reasons is speculated to be as follows.
Polarization treatment means that spontaneous polarization is arranged in a predetermined direction. This movement of the spontaneous polarization is achieved by two types of rotation, i.e., 180xc2x0 and 90xc2x0 rotation, on the whole. The residual component of the spontaneous polarization achieved by 90xc2x0 rotation has the property that it is restored to its original state by heating, while the residual component of the spontaneous polarization by 180xc2x0 rotation has the property that the polarization state is maintained below the Curie temperature. That is, the spontaneous polarization achieved by 180xc2x0 rotation is more stable to thermal shock as compared with the spontaneous polarization achieved by 90xc2x0 rotation.
FIGS. 1A, 1B, and 1C illustrate the change in directions of spontaneous polarization. FIG. 1A shows the directions of the spontaneous polarization before polarization treatment, which are different from each other. By the polarization treatment, all the spontaneous polarization is arranged in the same direction as shown in FIG. 1B. After this, by suffering a thermal shock, only the spontaneous polarization achieved by 90xc2x0 rotation is restored to the original state as shown in FIG. 1C.
A polarization treatment method has been proposed in which the spontaneous polarization of a piezoelectric material achieved by 90xc2x0 rotation is removed by aging treatment, utilizing the above-described properties, whereby the polarization degree is made up solely of the polarization degree achieved by 180xc2x0 rotation. (Japanese Unexamined Patent Publication No. 7-172914). As an aging treatment method for this case, a method (first method) of heat-aging at a temperature of from 200 to 250xc2x0 C., a method (second method) of repeating changing a temperature from xe2x88x9240xc2x0 C. to 130xc2x0 C., and then from 130xc2x0 C. to xe2x88x9240xc2x0 C. at about 50 cycles, a method (third method) of applying a compression stress of 50 MPa at about 100 cycles, and so forth are exemplified.
However, according to the conventional second and third aging treatment methods, since an excess load is applied to the piezoelectric material, there are problems that cause inconveniences such as cracking, breaking, or the like and the yield is reduced. On the other hand, according to the first aging treatment method, it is difficult to produce a piezoelectric material having a target polarization degree simply by carrying out heat-aging at a temperature of from 200 to 250xc2x0 C., though the yield is high. In particular, conventionally, the polarization-treatment is carried out at a voltage, time, and a temperature selected appropriately, and thereafter, the aging is conducted until the polarization achieved by 90xc2x0 rotation is completely removed. Therefore, obtained final polarization degrees are uneven, depending on how much the spontaneous polarization achieved by 180xc2x0 rotation is present on completion of the polarization treatment.
Accordingly, it is an object of the present invention to provide a method of polarization-treating a piezoelectric material whereby a target polarization can be attained at a high precision, and the treated piezoelectric material is stable thermally with passage of time.
The inventors carried out polarization and aging under different conditions. As a result, they have found that the ratio of the polarization degree xcex94f180 achieved by 180xc2x0 rotation to the polarization degree xcex94f90 achieved by 90xc2x0 rotation in the polarization degree xcex94f is dominated by the temperature only, irrespective of the polarization voltage and the polarization time. Therefore, when the temperature is predetermined, the ratio of the polarization degree xcex94f180 achieved by 180xc2x0 rotation to the polarization degree xcex94f90 achieved by 90xc2x0 rotation can be principally known.
Throughout this specification, the term xe2x80x9cpolarization degreexe2x80x9d refers to xe2x80x9cresidual polarization degreexe2x80x9d which is a polarization degree at an ordinary temperature after the polarization treatment is completed.
The polarization degree of a piezoelectric material can be expressed by use of a frequency difference xcex94f between the resonance frequency fr and the anti-resonance frequency fa of the piezoelectric material, as is well known.
FIG. 2 shows the relation between the polarization temperatures and the polarization degrees achieved by 180xc2x0 and 90xc2x0 rotation in the polarization degrees xcex94f, which were experimentally determined by the inventors of this invention. The above-mentioned polarization degrees xcex94f are obtained by carrying out the polarization (hereinafter, referred to as total-polarization) by which the maximum polarization degree can be achieved at each polarization temperature. In the experiment, as a piezoelectric material, PZT ceramics were used comprising Pb(ZrTi)O3 to which Sr, Cr were added.
For example, in the case such as in FIG. 2, the polarization temperatures T1 through T6 are 26xc2x0 C., 50xc2x0 C., 100xc2x0 C., 150xc2x0 C., 200xc2x0 C., and 250xc2x0 C., and the aging is carried out at the same temperatures as the polarization temperatures, respectively. The polarization degrees xcex94f180 achieved by 180xc2x0 rotation, after the aging, are constant, irrespective of the temperatures, while the polarization degrees xcex94f90 achieved by 90xc2x0 rotation are reduced with the temperatures being higher, and at the temperature T6, the residual polarization degree xcex94f is made up solely of the polarization degree achieved by 180xc2x0 rotation. With the polarization temperatures being changed from T1 to T6, the ratios of the polarization degrees achieved by 180xc2x0 rotation to those by 90xc2x0 rotation are changed from 77:23 to 78:22 to 81:19 to 85:15 to 90:10 to 100:0, for example. The ratios of the polarization degrees achieved by 180xc2x0 rotation to the polarization degree achieved by 90xc2x0 rotation depend on the temperatures only, and are not affected by the polarization voltages and the polarization times (total or half polarization). The polarization treatment may be carried out in the atmosphere or an insulating oil. Thus, the relation as shown in FIG. 3 is obtained. Herein, the term xe2x80x9chalf polarizationxe2x80x9d means the polarization before it reaches the total polarization, which is carried out while time and voltage are controlled.
To determine the ratios of the polarization degrees achieved by 180xc2x0 rotation to the polarization degrees achieved by 90xc2x0 rotation in the polarization degrees xcex94f, the (002) lattice plane of a piezoelectric material is measured by XRD (X-Ray Diffraction Method), and then determination is made as to the polarization temperature at which the polarization degree xcex94f made up solely of the polarization degree achieved by 180xc2x0 rotation can be obtained.
FIG. 4 shows the relation between the polarization temperatures and the X-ray intensities at the (002) crystal lattice plane. The X-ray intensities are obtained by the measurement of the piezoelectric material wherein the temperature has been restored to a room temperature after the polarization treatment. The term xe2x80x9cthe (002) lattice planexe2x80x9d means the lattice plane wherein the C axis is in parallel to the polarization direction. As seen in FIG. 4, the X-ray intensities at the (002) lattice plane are reduced with the polarization temperatures being raised. This shows that the polarization achieved by 90xc2x0 rotation is significantly restored. Further, at the temperature T6, the X-ray intensity at the (002) lattice plane is equal to that before the polarization is carried out. This shows that the polarization achieved by 90xc2x0 rotation has been completely restored. That is, the temperature T6 is the polarization temperature at which the polarization degree xcex94f made up solely of the polarization degree achieved by 180xc2x0 rotation can be obtained. Then, the ratios of the polarization degrees achieved by 180xc2x0 rotation to those achieved by 90xc2x0 rotation are determined as follows. First, polarization is carried out at plural appropriate temperatures which are lower than the temperature T6. Then, aging is conducted at the temperature T6. In this case, xcex94f, which is equal to a decrement caused by the aging at the temperature T6, is the polarization degree achieved by 90xc2x0 rotation. The remaining xcex94f is the polarization degree achieved by 180xc2x0 rotation. As a result the ratios of the polarization degrees achieved by 180xc2x0 rotation to those by 90xc2x0 rotation in the polarization degrees xcex94f as shown in FIG. 3. are obtained.
Based on the above-described knowledge, the inventors have devised methods of polarization-treating a piezoelectric material. According to the present invention, obtained is the maximum polarization degree xcex94f180max achieved by 180xc2x0 rotation which is determined by an employed piezoelectric material or the like. A target residual polarization degree xcex94f and the maximum polarization degree xcex94f180max are compared. Methods of polarization-treating a piezoelectric material according to first and second aspects of the present invention are applied in the case of xcex94fxe2x89xa6xcex94f180max shown as a result of the comparison. That is, these polarization methods are applied in the case that the polarization degree xcex94f made up solely of the polarization degree achieved by 180xc2x0 rotation can be obtained.
Herein, the expression xe2x80x9cthe polarization degree xcex94f made up solely of the polarization degree achieved by 180xc2x0 rotationxe2x80x9d is defined to include the polarization degree xcex94f which contains a polarization degree achieved by 90xc2x0 rotation to some degree, if any.
According to the first aspect of the present invention, first, a determination is made as to the temperature TA for obtaining the polarization degree xcex94f made up solely of the polarization degree achieved by 180xc2x0 rotation. The temperature TA can previously be determined experimentally. In general, the ratio of the polarization degree achieved by 180xc2x0 rotation becomes higher with the polarization temperature being increased. Next, the piezoelectric material is half-polarized at the temperature TA under the control of voltage and time so that the polarization degree xcex94f180 achieved by 180xc2x0 rotation becomes equal to the target polarization degree xcex94f. At the time when the half-polarization has been completed, the polarization degree includes the polarization degree achieved by 90xc2x0 rotation, in addition to the polarization degree achieved by 180xc2x0 rotation. The polarization achieved by 90xc2x0 rotation, produced by the half-polarization treatment, is completely restored to the original state by aging at the same temperature TA as that for the half-polarization treatment. As a result, the polarization degree made up solely of the polarization degree by 180xc2x0 rotation can be obtained. According to this method, a piezoelectric material which is stable thermally with passage of time can be obtained, since the residual polarization degree is made up solely of the polarization degree achieved by 180xc2x0 rotation with the polarization degree achieved by 90xc2x0 rotation being completely removed. Moreover, since the polarization degree xcex94f180 achieved by 180xc2x0 rotation is determined at the stage of the half-polarization, a piezoelectric material of which the polarization degree xcex94f (=xcex94f180) is correctly coincident with a target value can be obtained.
According to the second aspect of the present invention, similarly to the method according to the first aspect of the present invention, a determination is made as to the temperature TA for obtaining the polarization degree xcex94f made up solely of the polarization degree achieved by 180xc2x0 rotation. In the case that the target value xcex94f is achieved with difficulty at the temperature TA, half-polarization is carried out at a temperature TP, at which the polarization control can be easily performed, lower than the temperature TA under the control of voltage and time so that the polarization degree xcex94f180 achieved by 180xc2x0 rotation is equal to the target polarization degree xcex94f. On completion of the half-polarization, the polarization degree xcex94f90 achieved by 90xc2x0 rotation is included in the polarization degree xcex94f. After this, all the polarization achieved by 90xc2x0 rotation is restored to its original state by aging at the temperature TA. The polarization degree xcex94f180 achieved by 180xc2x0 rotation suffers no changes by the aging. By the above-described operations, similarly to the method according to the first aspect of the present invention, a piezoelectric material of which the residual polarization degree made up solely of the polarization degree achieved by 180xc2x0 rotation, and which is stable thermally with passage of time can be obtained.
According to third and fourth aspects of the present invention, a determination is made as to the maximum polarization degree xcex94f180max made up solely of the polarization degree achieved by 180xc2x0 rotation, which is determined by an employed piezoelectric material. A residual polarization degree xcex94f, which is a target value, and the maximum polarization degree xcex94f180max are compared. Methods according to the third and fourth aspects of the present invention are applied in the case of xcex94f greater than xcex94f180max shown as a result of the comparison. That is, in the case that the polarization degree xcex94f made up solely of the polarization degree achieved by 180xc2x0 rotation is achieved with difficulty by any treatment, these polarization methods are applied to obtain a polarization degree in which the ratio of the polarization degree achieved by 180xc2x0 rotation is highest. Herein, the above expression, xe2x80x9cthe polarization degree in which the ratio of the polarization degree achieved by 180xc2x0 rotation is highestxe2x80x9d is defined to include the polarization degree in which the ratio of the polarization degree achieved by 180xc2x0 rotation is lower to some degree than the highest ratio of the polarization degree achieved by 180xc2x0 rotation.
According to the third aspect of the present invention, the temperature TA for obtaining the polarization degree xcex94f of which the ratio of the polarization degree achieved by 180xc2x0 rotation is highest is determined. Next, total polarization is carried out at the temperature TA, and aging is conducted at the same temperature TA as that for the polarization treatment, whereby the polarization degree xcex94f of which the ratio of the polarization achieved by 180xc2x0 rotation is highest is obtained. According to this method, the polarization degree achieved by 90xc2x0 rotation is included in the residual polarization degree, but the ratio of the polarization degree achieved by 180xc2x0 rotation is highest. Accordingly, a piezoelectric material which is stable thermally with passage of time can be obtained. Moreover, a piezoelectric material of which the polarization degree xcex94f is coincident with a target value can be obtained.
According to the fourth aspect of the present invention, similarly to the method according to the third aspect of the present invention, a determination is made as to the temperature TA for obtaining the polarization degree xcex94f in which the ratio of the polarization degree A180 achieved by 180xc2x0 rotation to the polarization degree xcex9490xc2x0 achieved by 90xc2x0 rotation has a predetermined value. In the case that total polarization is carried out with difficulty, to obtain the above-described polarization degree xcex94f, the total polarization is carried out at a temperature TP lower than the temperature TA, at which the total polarization can be conducted relatively easily. Next, by aging at the temperature TA, the polarization achieved by 90xc2x0 rotation is partially restored so that the ratio of the polarization degree achieved by 180xc2x0 rotation to that by 90xc2x0 rotation has a predetermined value. By the above-described operations, similarly to the method according to the third aspect of the present invention, a piezoelectric material of which the ratio of the polarization degree achieved by 180xc2x0 rotation to that achieved by 90xc2x0 rotation has a predetermined value, and moreover, the overall polarization degree xcex94f is correctly coincident with a target value can be obtained.